Computers commonly use hard disc drives to store large amounts of data in a form that can be readily accessed by a user. A disc drive generally includes a stack of vertically spaced magnetic discs that are rotated at a constant high speed by a spindle motor. The surface of each disc is divided into a series of concentric, radially spaced data tracks in which the data is stored in the form of magnetic flux transitions. Typically, each data track is divided into a number of data sectors that store data blocks of a fixed size.
Data are stored and accessed on the discs by an array of read/write heads mounted to a rotary actuator assembly, which is also called an "E-block." Typically, the E-block includes a plurality of actuator arms which project outwardly from an actuator body to form a stack of vertically spaced actuator arms. The stacked discs and arms are configured so that the surfaces of the stacked discs are accessible to the heads mounted on the complementary stack of actuator arms. Head wires included on the E-block conduct electrical signals from the heads to a flex circuit, which in turn conducts the electrical signals to a flex circuit bracket mounted to a disc drive basedeck. For a general discussion of E-block assembly techniques, see U.S. Pat. No. 5,404,636 entitled METHOD OF ASSEMBLING A DISK DRIVE ACTUATOR, issued Apr. 11, 1995 to Stefansky et al., assigned to the assignee of the present invention.
The actuator body pivots about a cartridge bearing assembly which is mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The actuator assembly includes a voice coil motor which enables the actuator arms and the heads attached thereto to be rotated about the cartridge bearing assembly so that the arms move in a plane parallel to the surfaces of the discs to selectively position a head over a preselected data track.
The voice coil motor includes a coil mounted radially outwardly from the cartridge bearing assembly, the coil being immersed in the magnetic field of a magnetic circuit of the voice coil motor. The magnetic circuit comprises one or more permanent magnets and magnetically permeable pole pieces. When current is passed through the coil, an electromagnetic field is established which interacts with the magnetic field of the magnetic circuit so that the coil moves in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces.
Each of the heads is mounted to an actuator arm by a flexure which attaches to the end of the actuator arm. Each head includes an interactive element such as a magnetic transducer which either senses the magnetic transitions on a selected data track to read the data stored on the track, or transmits an electrical signal that induces magnetic transitions on the selected data track to write data to the data track. Air currents are caused by the high speed rotation of the discs. A slider assembly included on each head has an air bearing surface which interacts with the air currents to cause the head to fly at a short distance above the data tracks on the disc surface.
A continuing trend in the industry is the reduction in the size of modern disc drives. As a result, the discs in the disc stacks of modern disc drives are increasingly being brought closer together, providing narrower vertical gaps between adjacent discs. This trend toward decreasing size is driving the industry toward smaller heads, longer and thinner actuator arms and thinner gimbal assemblies. Although these and other size reductions facilitate greater storage capacity, such narrow vertical spacing of the discs and thinning of the actuator arms gives rise to a problem of increased sensitivity of the disc drives to non-operating mechanical shocks.
Non-operating mechanical shocks can cause significant deflection of the discs, leading to catastrophic damage to the disc media and heads. More particularly, disc to arm contact can induce a shock wave large enough to travel down to the flexure assemblies and heads, causing the heads to lift off of the disc surfaces as a result of the relatively flexible flexure assemblies to which the heads are attached. The heads can thus obtain significant velocities as they accelerate away from and then back toward the discs. When such velocities are sufficiently severe, damage can occur to the heads and the surfaces of the discs as the heads strike the landing zone portions of the discs. Moreover, should a head tilt during such liftoff, a corner of the head can strike the disc surface, increasing the probability of damage to the head or the disc.
That is, such non-operating mechanical shocks, often encountered during shipping and other handling of disc drives, can cause the actuator arm tips to contact the media on the discs. When a non-operating shock is encountered, the discs and actuator arms vibrate, causing displacement of the actuator arms and discs in a vertical direction (often referred to in the industry as the "Z-axis"). The discs and actuator arms oscillate about the positions held prior to encountering the non-operating shock. Because the actuator arms have different characteristics than the discs, the oscillation of the actuator arms occurs at a different frequency than the oscillation of the discs. As a result, the wavefunctions for the oscillations of the actuator arms and discs become out of phase, which means that the actuator arms and discs move either toward or away from each other. If the amplitude of the displacement is large enough, the tips of the actuator arms contact the discs. The resulting contact, which can damage the surfaces of the discs, the heads and the actuator arms, has been recognized as a predominant failure mode in modern disc drives.
Although little can be done to completely eliminate contact between the actuator assembly (the heads, actuator arms or both) and the discs of a disc drive, it is desirable to reduce the amplitude of the force of impact as well as the number of impacts which are encountered for any non-operational shock. Thus, there is a need for an improved approach to reducing the susceptibility of damage in disc drives resulting from non-operating mechanical shocks.